Holmium-lasers (Ho-lasers) emitting laser beams having a wavelength of 2.1 μm are suitable for various medical applications, especially in minimal-invasive or endoscopic surgery. A wavelength of 2.1 μm and the pulsed emission characteristics of Ho-lasers (e.g. a Holmium-Yttrium-Aluminium-Garnet-laser, Ho:YAG-laser) are suitable for precisely localized tissue ablation without conflicting thermal side effects in surrounding volume.
The capacity of precisely localized treatments and the confinement of radiation energy within an area of interest is determined by two facts:                Water has a strong absorption coefficient at a wavelength of 2.1 μm wherein the water absorption coefficient characterizes tissue ablation characteristics because human tissue contains about 80% water (see FIG. 1).        High transmission rate of a laser beam having a wavelength of 2.1 μm through silica fibers having a low or no water content up to a treatment region.        
Known Ho-laser systems for minimal-invasive surgery comprise a Ho-laser, a transmission fiber for transmission of the laser beam from the Ho-laser, a coupling unit coupling the Ho-laser and the transmission fiber and an end piece connected to the transmission fiber for delivering the laser beam to a treatment region.
Known Ho-laser systems for minimal-invasive applications provide laser beams having a power of 20 W and more. A typical transmission fiber has a diameter of 400 μm and more for use, e.g., in the treatment of Benign Prostate Hyperopia (BPH) and lithotripsy.
Known systems however exhibit several problems. Coupling of pulsed high-energy laser radiation having a wavelength of 2.1 μm into a transmission fiber having a small diameter is critical and troublesome and results in maintenance problems and degradation of involved components. Ho-laser systems require extensive cooling and space for system accommodation. Further, pulsed laser radiation provided by Ho-laser systems is not suitable for several minimal-invasive applications due to shock waves produced by pulsed laser radiation.